Autism Risk May Increase If Child's Mother Has High DDT Exposure

Pregnant women who have high levels of a byproduct from the banned insecticide DDT in their bodies may be more likely to have a child with autism, a new study finds.

Researchers looked at nearly 800 mothers from Finland who had children diagnosed with autism spectrum disorder (ASD) and found that these women had higher levels of DDE, a byproduct of DDT, than did mothers whose children did not have ASD.

The finding sheds light on another potential cause of autism, a neurodevelopmental condition that affects communication, behavior and the ability to interact with others. But DDE is likely just "one piece of a puzzle," said study lead researcher Dr. Alan Brown, a professor of psychiatry and epidemiology at Columbia University Medical Center in New York City. [8 Ways That Air Pollution Can Harm Your Health]

"Very likely, you need other predisposing factors [for autism] in addition to [DDE]," Brown told Live Science. "I don't think moms should be going out and getting tested for these things."

To do the study, Brown partnered with researchers in Finland, a country with a universal health care system that tracks diagnoses, such as autism, in all people living there. The researchers identified 778 cases of children diagnosed with autism who were born from 1987 to 2005, then matched those individuals with controls — that is, children who were born during that same period but did not have an autism diagnosis.

Then, the researchers analyzed blood samples that had been taken from these children's mothers during early pregnancy. After analyzing these blood samples for DDE, the researchers found that mothers with higher levels of the compound were more likely to have children with autism. What's more, the DDE levels were even higher in mothers whose children had both autism and an intellectual disability, Brown said.

To be specific, the odds have having a child with autism were 32 percent greater in the women with higher DDE levels compared with the women with lower DDE levels, Brown said. In addition, the odds of having a child with autism and an intellectual disability were increased by more than twofold when comparing the women whose DDE levels were in the top 25 percent of the group with those who had low DDE levels.

The finding held even when the researchers controlled for several factors, including the age of the mother, the mother's socioeconomic status and whether the parents had a history of psychiatric disorders.

The researchers also tested the mothers' blood samples for PCBs (polychlorinated biphenyls), another class of environmental pollutants, but found that these substances were not associated with autism risk.

PCBs and DDT have both been banned for more than 30 years in many countries, including the United States and Finland. But because these chemicals break down very slowly, they stick around in the environment and the food chain. "What happened was DDT was sprayed in the air as an insecticide to kill mosquitoes, to kill bugs on fruits and vegetables," Brown said. "Then, it got into the soil and groundwater."

Virtually everybody has some level of DDT and PCBs in their body. When the body metabolizes DDT, the chemical breaks down into DDE, Brown said. When a woman is pregnant, the fetus is exposed to even higher levels of these chemicals than the mother is exposed to, "because they kind of get concentrated when they go through the mom's blood to the placenta," he said. "Then, they get into the fetal brain, and they alter fetal brain development." [11 Facts Every Parent Should Know About Their Baby's Brain]

Brown recommended that women who are pregnant or thinking about getting pregnant eat organic fruits and vegetables, as well as wash produce to rinse off toxic residues, but "I wouldn't say it's cause for alarm," Brown said. "We showed that overall in autism, there was a modest increase in risk [from DDE], but the vast majority of offspring who are exposed to the high levels still won't get autism."

As mentioned, DDE may just be one piece of the puzzle for explaining what causes autism. Hundreds of other studies show that additional factors also play a role, including low birth weight, the age of the parents (older parents tend to have a greater risk), whether the mother takes the anticonvulsant valproic acid while pregnant, whether the parents have a close relative with autism and whether the mother has increased levels of inflammation while pregnant.

But this study may play an important role when politicians craft public policy, at least when it comes to using certain chemicals in the environment, Brown said.

That idea was echoed by Marc Weisskopf, a professor of environmental epidemiology and physiology at the Harvard T.H. Chan School of Public Health, who was not involved in the new research.

"DDT is very long-lived in the body, so a woman with high levels may not be able to do much about it at the time she starts thinking of getting pregnant," Weisskopf told Live Science in an email. "For any individual mother, I would still stress that the absolute increase in risk from such exposure still certainly remains small. From a larger societal point of view, it is more evidence to try and limit DDT exposures overall."

The study is the first to connect an insecticide with risk for autism by testing a mother's blood sample. It was published online today (Aug. 16) in the American Journal of Psychiatry.

As a senior writer for Live Science, Laura Geggel covers general science, including the environment, archaeology and amazing animals. She has written for The New York Times, Scholastic, Popular Science and Spectrum, a site covering autism research. Laura grew up in Seattle and studied English literature and psychology at Washington University in St. Louis before completing her graduate degree in science writing at NYU. When not writing, you'll find Laura playing Ultimate Frisbee. Follow Laura on Google+.